1. Field of the Invention
The present invention relates generally to fusers and electrophotographic printing devices, and, more particularly, to release mechanisms that unload the nip formed by the pressure roll against the hot roll in the fuser.
2. Description of the Related Art
In the electrophotographic (EP) imaging process used in printers, copiers and the like, a photosensitive member, such as a photoconductive drum or belt, is uniformly charged over an outer surface. An electrostatic latent image is formed by selectively exposing the uniformly charged surface of the photosensitive member. Toner particles are applied to the electrostatic latent image, and thereafter the toner image is transferred to the media intended to receive the final permanent image. The toner image is fixed to the media by the application of heat and pressure in a fuser.
A fuser is known to include a heated roll and a pressure roll, each covered with a compliant outer covering. The pressure roll is loaded against the heated roll by means of a spring-loaded bell crank to create a nip region for fusing the toner.
As color printers have become smaller in size, to reduce cost and to minimize space requirements in offices, the paper path through the printer has become shorter and shorter. As a result, if a paper jam or other process interruption occurs while a portion of the media is still in the fuser nip, the media is likely trapped in at least one and as many as four transfer nips. In some vertical machine architectures, the PC drum to transfer roll nip is located in the front of the machine. As the door of the machine is opened, the media is caught in nips between rolls moving with the opening cover and the fuser nip. The fuser nip pressure is normally higher than even a combination of several transfer nips. The media is likely pulled out of the transfer nips rapidly as the door is opened, which could cause damage to the PC drums or even cause the PC drums to be pulled off the front door. The PC drum is unloaded when the door is fully opened, but for the first part of the door motion the PC drum is still loaded against the transfer rolls. The paper will tug on the PC drum and slip in the transfer nips, possibly scratching the drums or even pulling them from their located features in the front door.
Several attempts are known in the prior art to overcome this problem. Manual levers have been used on the fuser module to relieve the fuser nip load. This requires an additional action by the user, and is not desirable. Further, because the fuser load is high, significant mechanical advantage is required, which results in a large, unsightly fuser nip release lever. Since opening the cover does not automatically release the fuser nip, it is possible that a user could open the cover and attempt to remove a jam without releasing the fuser nip lever potentially causing damage to the machine.
It is known also to employ a mechanism whereby opening one cover causes a part in the base machine to push on the fuser bell crank, thus releasing the fuser nip. In designs of this type it is common that the front door cannot be opened unless the cover that releases the fuser nip is opened first. A disadvantage of this design is that the base machine applies considerable load to the fuser module, which makes inserting or removing the fuser module difficult for the user.
A further attempted solution has been to provide a mechanism on board the fuser module itself that opens and closes the fuser nip. The actuation of the mechanism is also accomplished internal to the fuser itself. Usually this is achieved through a gear train and cam acting on the spring loaded bell cranks, which could be powered by a separate motor or by reversing the fuser motor. A disadvantage of this design is that the mechanism is expensive and may require a separate motor in the case of printers in which the fuser motor is dedicated to driving the fuser rolls forward and backward in a duplexing arrangement.
What is needed in the art is a simple, inexpensive and effective structure to automatically unload the fuser nip and to retain the fuser nip in an unloaded position during servicing.